The objectives of this research are: (1) Cultivation of Mycobacterium leprae in tissue culture. (2) Utilization of the neonatally thymectomized Lewis rat (NTLR) as a model for the development and evaluation of drug regimens in the treatment of leprosy. (3) Development in the NTLR of a model of persistence of M. leprae in patients with multibacillary leprosy. (4) Demonstration of the capacity of the NTLR to act as a host for detecting small numbers of persisting M. leprae in the presence of large numbers of dead M. leprae, such as one finds in the treated lepromatous patient. (5) The investigation of the nude athymic rat as a possible replacement for the NTLR in objectives (2) to (4).